The ability of a radar system to distinguish between two closely spaced targets is termed resolution. Resolution is further broken down into range resolution and angular (azimuth or elevation) resolution. Range resolution is the ability of the radar system to distinguish between two targets on the same azimuth and closely spaced in range. Angular resolution is the ability of the radar system to distinguish between two targets at the same range and closely separated in azimuth or elevation. Angular resolution is directly related to radar antenna beamwidth. Specifically, in an incoherent pulse radar system, multiple targets separated by the radar antenna beamwidth are generally considered to be inseparable; that is, when two targets are located at the same range but X degrees apart, the radar system cannot resolve the targets if the radar antenna employed has a -3 db beamwidth of X degrees. In this case what are actually two targets will be displayed as a single target by the radar system.
The steady steep drop over the last decade in the cost per bit of solid state digital logic components, together with the fact that solid state digital logic components allow large amounts of data capacity to be compacted into an extremely small volume at a very small weight, have led to the marketing of devices wherein analog data is converted into digital format and digitally processed before being converted back into analog format for use. For example, airborne digital radar systems have a major portion of the airborne radar market. In these systems, a radar return signal in analog format is converted into a digital format. The digitized return signals are integrated scan to scan and pulse to pulse to remove noise and other anomalies from the signal before display or transmission to a remote location for use.
The art has long taught the use of a digital reiteration memory which is periodically updated with new data and which includes in memory all the data required for a single frame of a cathode ray tube display. The reiteration memory is continuously read-out at flicker-free rates, synchronously with the cathode ray tube raster scan period. A digital reiteration memory is used in some radar systems to store a complete frame of radar data to be read-out onto a cathode-ray tube as described above. In this case the radar return signals are digitized and optionally integrated before storage into the reiteration memory.
In a standard search type radar system an antenna beam is made to sweep either in azimuth or elevation from a common center through a field of interest. In a pulse radar the sweep is actually comprised of a plurality of electromagnetic pulses radiated from the antenna at closely spaced angular intervals. The return from each pulse comprises a radar return signal or line of data which describes the space through which the radar pulse travels. The radar pulse actually illuminates an angular sector of space with electromagnetic energy with the highest energy content being generally along the antenna boresight and tapering to either side of boresight. The angular sector is defined by the radar antenna beamwidth and is normally considered to be contained within the half power or -3 db beamwidth of the radar antenna. As previously mentioned, the radar returns from multiple targets at approximately the same range and lying within the -3 db beamwidth cannot usually be resolved into individual targets.